1. Field of the Invention
The invention relates to a linear displacement detecting structure and an electronic component mounting head including the linear displacement detecting structure.
2. Description of the Related Art
There has been known various linear displacement detecting structures, each structure including a shaft hole member having a shaft hole formed thereon, a shaft supported movably in its axis direction on the shaft hole member through the shaft hole, a scale moving with the shaft united, and a sensor for detecting the scale to determine the relative position in the axis direction between the shaft and the shaft hole member.
The shaft is slidable in the shaft hole member in the axis direction with spline fitting or the like.
A non-contact sensor, such as an optical sensor or magnetic sensor, is generally employed for detecting the scale.
The scale is formed by a number of fine grooves or magnetic thin film or the like arranged in a fine pitch along the axis direction. Abrasion of the scale may cause a degradation of the detecting precision by the sensor, and sometimes a detection failure. Therefore, the scale is usually arranged apart from the shaft hole member so as not to slide on to the shaft hole member.
FIG. 10 shows an example of conventional linear displacement detecting structure.
The linear displacement detecting structure 100 includes a shaft hole member 104 having a shaft hole 102 formed thereon, a shaft 106 supported movably in its axis direction on the shaft hole member 104 through the shaft hole 102, a scale 108 moving with the shaft 106 united, a sensor 110 for detecting the scale 108, and a base 112 supporting the sensor 110 and the hole member 104.
The shaft 106 projects out from the end of cylindrical base 112 coaxial with the shaft, and protrudes or retracts in the axis direction.
The scale 108, facing the sensor 110, is a rod disposed in parallel with the shaft 106 in the vicinity of and along the periphery of the base 112, and has on the surface a number of fine grooves formed in the axis direction by a proper fine pitch.
One end of the scale 108 is fixed to the shaft 106 through a plate bracket 114 disposed perpendicular to (radial direction) the shaft 106.
The bracket 114 comes close to the base 112 when the shaft 106 is at the most retracted position. With such arrangement that the scale 108 and the bracket 114 are disposed close to the base 112, protruded amount of the scale and the like is restricted to be small-sized.
The sensor 110, a reflection-type photo-sensor, can detect the position of the scale 108 or the shaft 106 in the axis direction, sensing the grooves on the scale 108.
Since the sensor 110 detects the position of the shaft 106 indirectly through the scale 108 and the bracket 114, a detection error arises due to errors in fabricating and assembling the scale 108 and the bracket 114.
Since the scale 108 and the bracket 114 are required to be light weighted for cooperative movement with the shaft 106, it is difficult for them to have high rigidity. Their low rigidity causes the bracket 114 to pivotally swing around the mounting spot slightly, whereby the movement of the scale 108 does not synchronize with that of the shaft 106, resulting in a detection error of the sensor 110.
The shaft 106 may be extended to a protrusion direction to directly attach a scale to the extended portion with a sensor disposed near the extended portion. This structure can avoid the above-described detection error, as well as prevent the abrasion of the scale.
However, this extension of the shaft 106 is not preferable in view of installation space of the device because it makes the device longer in the axis direction.
For example, as JPA 2001-3008811 discloses, an electronic component mounting head having a linear displacement detecting structure needs compact size, avoiding long size in the axis direction.
It is an object of the invention to provide a linear displacement detecting structure in smaller size with higher detecting precision, and an electronic component mounting head having the linear displacement detecting structure.
In order to achieve the object mentioned above, the invention provides for a linear displacement detecting structure and an electronic component mounting head having the detecting structure. In a structure aspect, the invention includes a shaft hole member having a shaft hole formed thereon, a shaft supported movably in its axis direction on the shaft hole member through the shaft hole, a scale moving with the shaft united, and a sensor for detecting the scale to determine the relative position in the axis direction between the shaft and the shaft hole member. The structure has a contact area where a part of the periphery of the shaft fits in/contacts the shaft hole member directly or indirectly, and a non-contact area where the other part of the periphery is spaced apart from the shaft hole member. The scale is disposed in the non-contact area, and the sensor is disposed in the vicinity of the shaft hole member.
In a preferred embodiment, the shaft fits in the shaft hole member with a ball-spline mechanism.
In a particular arrangement, the structure may further include a bearing for supporting the shaft hole member rotatable around its axis so as to rotate the shaft united with the shaft hole member.
In a mounting head aspect, the mounting head includes the shaft hole member, a cylindrical shaft supported assembly in its axis direction on the shaft hole member through the shaft hole and having a component pick-up nozzle at the near top end for absorbing the component, the bearing, the scale, and the sensor. This structure detects the relative position in the axis direction between the shaft and the shaft hole member.
According to the invention, the relative position in the axis direction between the shaft and the shaft hole member can be detected in high precision.